This invention relates to a two-component color forming type recording material having absorptions particularly at near infrared portion and giving developed color images excellent in storability.
As recording materials applying a two-component color forming system comprising a colorless or light-colored dye precursor and a color developer which can form a color in contact with the dye precursor, there are well known pressure-sensitive recording materials, and heat-sensitive recording materials. Demand for these materials increases year after year. Further, electric current-passing heat-sensitive recording materials, light-sensitive recording materials, heat transfer recording materials are also developed recently.
Heat-sensitive recording materials generally comprise a support and formed thereon a color forming layer containing as major components an electron donative colorless or light-colored dye precursor and an electron-accepting color developer. Upon heating with a thermal head, a thermal pen, a laser light, or the like, the dye precursor and the color developer react instantly to give a recording image (e.g. Japanese Patent Examined Publication Nos. 43-4160 and 45-14039). Since these heat-sensitive recording materials have advantages in that recording can be obtained by using a relatively simple apparatus, the maintenance is easy and no noise is produced, they are widely used in recorders for measuring, facsimile machines, printers, computer terminals, labels, automatic ticket vending machines, etc.
Particularly, heat-sensitive recording labels are used as bar code labels for point of sales (POS) system, price labels, delivery and skipping labels and the like, wherein adhering is necessary after printing. The POS system remarkably spreads in large-scale stores such as supermarkets and department stores as well as chain stores of special stores, restaurants, etc. In order to grasp the needs of consumers clearly and to plan reasonable business strategy, it is an object to totalize and analyze sales information of goods. In order to attain such an object, methods for reading bar codes and OCR letters adhered to goods sold in stores with a scanner optically have been developed rapidly. As a method for printing bar codes, etc., there is widely used a heat-sensitive recording method which has advantages in that miniaturization and simplification of the machine is possible, speed up of recording is possible, there is no contamination of hands and goods by the ink, and there is no fear of blur of ink.
As a method for reading developed color images such as bar codes, etc., there has been used a He.Ne laser light having a red color at a wavelength of 633 nm. But recently, the use of a semiconductor laser is widely spreading. This is because it has many advantages in that the modulation can be made directly by an electric current, miniaturization is possible, the use of semiconductor laser is easy with a low cost, erroneous operation due to contamination is rare because the oscillation wavelength is in the near infrared portion of 700 to 1500 nm. Therefore, a demand for reading developed color images in heat-sensitive recording labels by using the semiconductor laser increases. According to known two-component type heat-sensitive recording materials comprising a dye precursor and a color developer, the absorption wavelength of developed color images is, even in a black color-forming system, at a longer wavelength side of 550 to 620 nm which range can be read by a He.Ne laser light but cannot be read by using a laser light having a wavelength at the near infrared portion. Some of the present inventors have provided heat-sensitive recording materials readable by a laser light at the near infrared portion (e.g. Japanese Patent Unexamined Publication Nos. 61-175077, 61-228985, 61 -228986, etc.), but they are still insufficient in storability of developed color images, particularly insufficient in light resistance.